The force applied by the student to the ball is 450 N
It is required to find the force the ball apply on the student. Here, Newton's third law of motion works. The force applied by the student to the ball is equal in magnitude with the force that the ball apply on the student. Hence, the force of 450 N is ball applied on the student.
The force the box is exerting on Manuel is the weight of the box, downward: and this force is perfectly balanced by the constraint reaction applied by Manuel's hand, pushing upward.
